The present invention generally relates to the field of light sources. More specifically, the present invention relates to a method of improving light extraction from a light emitting diode (xe2x80x9cLEDxe2x80x9d) of flip-chip design through geometrical shaping of the chip.
An LED is a semiconductor chip or die that emits light when a forward current flows through the LED. FIG. 1 illustrates a known LED 30 of flip-chip design, where the LED 30 includes a transparent substrate 31 having a cuboidal shape (e.g., a rectangular prism), a negatively doped layer 32, an active p-n junction layer 33, and a positively doped layer 34. A pair of ohmic contacts 35 and 36 are employed to forward bias LED 30 whereby light is generated and emitted from the active p-n junction layer 33 into the substrate 31. Typically, a refractive index of the substrate 31 will be different than a refractive index of the surrounding environment (e.g., air or a medium employed to encapsulate the substrate 31). As illustrated in FIG. 2, such a difference in the refractive indices establishes a critical angle that results in a partitioning of the substrate 31 into escape cones 31a, 31c and 31e and total internal reflection cones (xe2x80x9cTIRCxe2x80x9d) 31b and 31d. It is well known in the art that the substrate 31 will emit light from the escape cones 31a, 31c and 31e as exemplified by the arrows therein, because an angle of incidence of the emitted light will be less than the critical angle. Conversely, the substrate 31 will not emit light from the TIRC cones 31b and 31d as exemplified by the arrows therein, because an angle of incidence of the trapped light will be equal to or greater than the critical angle for each internal reflection of the trapped light by the substrate 31.
The present invention advances the art by a contribution of an improved light extraction from a LED of flip-chip design.
A flip-chip design of a LED is constructed in accordance with various embodiments of the present invention. The LED comprises a light emitting region including a negatively doped layer, a positively doped layer, and an active p-n junction layer between the negatively doped layer and the positively doped layer. The LED further includes a transparent substrate overlying the light emitting region, wherein the transparent substrate has a pyramidal shape determined by (1) the composition of electrically conductive or electrically non-conductive material, (2) the number of side surfaces, (3) the degree of offset of an apex or top surface, and (4) the slope angle of each side surface.
The features and advantages of the present invention will become further apparent from the following detailed description of the presently preferred embodiments, read in conjunction with the accompanying drawings. The detailed description and drawings are merely illustrative of the present invention rather than limiting, the scope of the present invention being defined by the appended claims and equivalents thereof.